This Week in Science

Science  16 Mar 2001:
Vol. 291, Issue 5511, pp. 2043
  1. Smoke and Ozone

    Smoke aerosol and ozone are both produced by biomass burning. A strong link between elevated tropospheric ozone and biomass burning has been shown particularly for southern Africa, but other studies have indicated that tropospheric ozone is driven by large-scale atmospheric dynamics and by lightning. Thompson et al. (p. 2128) have analyzed daily satellite measurements of aerosol and ozone in the troposphere for August 1996 to December 1998 and show that tropospheric ozone increased well before the large fires in Indonesia connected with El Niño. Comparison of these data with earlier records from the 1980s shows that smoke and ozone signals were generally decoupled, indicating that biomass burning is only one factor determining tropical tropospheric ozone.

  2. Water Workings

    CREDIT: P. L. GEISSLER ET AL.

    Ions may dissolve readily in water, which itself autoionizes into H3O+ and OH ions. Two reports focus on the complexity underlying these apparently simple processes (see the Perspective by Klein). Water forms solvation shells around dissolved ions, but gaining detailed insights into the structure and properties of these shells is difficult because their signature cannot easily be separated from that of the surrounding bulk water. Kropman and Bakker (p. 2118) have used nonlinear mid-infrared spectroscopy overcome this problem and find that the motion of the solvation shell is sluggish compared to that of the bulk liquid—removing a water molecule causes the shell to break down and is thus associated with a high energy cost. In contrast, breaking of a hydrogen bond in the bulk can be compensated easily by the simultaneous formation of a new one. Another well-known property of water, its autoionization that governs its pH constant, has been even more intractable at the molecular level. Some proportion of water molecules dissociates in liquid water, but individual events are rare. Geissler et al. (p. 2121) combined high-accuracy ab initio molecular dynamics calculations with a method for efficient sampling of the liquid water potential energy surface. They then elucidated a mechanism of autoionization that combines fluctuations in the long-range electrostatic interactions with hydrogen-bond dynamics. Ions produced in this way usually recombine quickly, but when the fluctuation coincides with the breaking of the hydrogen-bond wire, the system crosses a transition state and rapid recombination cannot take place.

  3. Controlling Cobalt Nanoparticles

    Recently a strategy has been described for obtaining highly uniform nanoparticles, called size-distribution focusing, in which the nucleation and growth processes are separated during synthesis. Combined with the use of surfactants that bind preferentially to different growth faces, the size and shape of CdSe particles could be controlled. Puntes et al. (p. 2115) now report that this approach can be applied to the synthesis of single-crystalline Co nanoparticles and nanorods, which are of special interest for their size- and shape-dependent magnetic properties. These magnetic particles organize into two- and three-dimensional superstructures.

  4. Shuttle Snapshot

    Several examples of molecular “shuttles” have been reported in which a molecular ring threaded onto a linear molecule can move back and forth between different positions. Brouwer et al. (p. 2124; see the Perspective by Sauvage) studied the dynamics of the cycling of a rotaxane molecule between two different hydrogen-bonded “stations” driven by pulsed-laser excitation. In acetonitrile solvent, transient optical absorption spectroscopy shows that the photon-induced motion occurs in about 1 microsecond. Charge recombination causes the rotaxane to shuttle back, a process that takes about 100 microseconds.

  5. Two-Way Trade

    Mitochondria, the powerhouses of the cell, are semi-autonomous organelles that possess their own limited genome. Much is known about the import of proteins manufactured in the cytosol into these organelles. Young et al. (p. 2135) now report an unexpected observation—mitochondria from the yeast Saccharomyces cerevisiae can export peptides through a membrane peptide transporter. The role of peptide export is still unclear but may potentially allow communication between mitochondria and the rest of the cell.

  6. Avoiding the Noon Day Sun

    Chloroplasts of the plant Arabidopsis convert light to metabolic energy—up to a point. When the incident light is simply too intense, the chloroplasts shuffle off to the sides of the cell to find some shade near the adjacent cells walls. Under more moderate light conditions, the chloroplasts spread out over the face of the cell to capture as many photons as possible. Kagawa et al. (p. 2138; see the cover) have now identified a protein that helps regulate this process, NPL1, that resembles a known blue-light photoreceptor. Thus, the plant appears to fine-tune and protect its light-harvesting capabilities by using photoreceptors to direct subcellular compartmentalization.

  7. Calling in Your Allies

    When attacked by herbivores, some plants are known to emit volatile compounds that attract predators of the herbivores. However, these indirect defenses have been demonstrated only in artificial laboratory and agricultural situations. Kessler and Baldwin (p. 2141; see the Perspective by Sabelis et al.) show that such systems also operate under natural conditions. Using plants of Nicotiana attenuata growing in the Great Basin Desert in Utah, they directly manipulated individual components of the suite of volatile organic compounds released after herbivore attack by Manduca caterpillars. They identified compounds that dramatically increased attack by the caterpillar's predators and that also reduced the oviposition rate of the Manduca moth. Thus, the plant can exert both “bottom-up” and “top-down” control of its enemies.

  8. Doing More Than Locomotion

    The major sperm cytoskeletal protein (MSP) functions in the crucial role of locomotion. Miller et al. (p. 2144; see the Perspective by Villeneuve) found that in the nematode, MSP also executes another critical step in reproduction—inducing the female's oocyte to come out of the arrest phase and mature. These results suggest that other proteins with MSP-like domains, which are found in plants, fungi, and other animals, may also have a signaling role.

  9. Brains Take Shape

    CREDIT: S. AGARWALA ET AL.

    What gives a tissue or organ its stereotypic size, shape, and orientation? Experiments by Agarwala et al. (p. 2147) now help to elucidate the molecular mechanism underlying the formation of different brain tissues. They altered the size, shape, and orientation of the Sonic Hedgehog morphogen in the developing chick brain and show that this signaling molecule can control the size, shape, and orientation of characteristic brain structures.

  10. Editing Is Essential

    In the process known as RNA editing, some messenger RNAs are chemically modified after they are synthesized and before they are translated into proteins. Editing was first discovered in the mitochondrial RNAs of trypanosomes, in which a large protein complex catalyzes the insertion and deletion of several uridine residues, which are specified by small guide RNA molecules. Schnaufer et al. (p. 2159) have demonstrated that a component of this complex is an RNA ligase that functions during the bloodstream phase of the parasite life cycle and that repression of this ligase results in parasite death, which suggests that inhibitors of this complex may be useful chemotherapeutic agents.

  11. Mixed Signals

    Multiple signaling pathways converge on the transcription factor NF-κ, which results in the expression of a range of genes. One of the upstream kinases that can activate NF-κB is NF-κ-inducing kinase (NIK), an enzyme that has been implicated in the selective gene expression in response to discrete signals received at the cell surface. Yin et al. (p. 2162) studied cytokine-induced transcriptional activity of NF-κ in mice carrying a targeted disruption of the NIK locus and observed the selective loss of transcription, but not NF-κ-DNA binding, in response to signals delivered through the lymphotoxin-αβreceptor (LTαβ-R). This effect, which was not seen in response to related cytokines, namely tumor necrosis factor and interleukin-1, correlated directly with disruption to the development of lymphoid tissue already reported in mice deficient in LTαβ-R.

  12. Cultural Influences on Reading

    Dyslexia is a complex disorder that causes different degrees of impairment in reading and that also varies in prevalence across cultures. Paulesu et al. (p. 2165; see the news story by Helmuth) have undertaken a cross-cultural study of dyslexic individuals in Italy, France, and the United Kingdom by utilizing behavioral tests and brain imaging scans. They confirm earlier findings that languages with shallow orthographies (where letters map onto sounds in a one-to-one manner), such as Italian, result in less severe impairment. Nevertheless, the underlying neural activation patterns are consistent across dyslexic subjects in all three countries: There is reduced activity in the left temporal cortex, which suggests there may be fewer or less stereotyped connections among brain regions than is observed during reading in normal individuals.

  13. All Together Now?

    Regional changes in climate during “abrupt” climate transitions have not all occurred simultaneously. For example, interstadial and deglacial warming in Antarctica normally has preceded Arctic warming by about 2000 or 3000 years. How regional climates lead and follow over the rest of the world is still generally poorly understood, however. One of the central questions is whether the tropics warmed before the high northern latitudes or at the same time, in part because different climate records give different answers. Kienast et al. (p. 2132) present data from the tropical South China Sea which show that sea surface warming there happened synchronously with the surface air temperature increase that occurred in Greenland about 14,600 years ago during the Bølling Transition. These results seem to reinforce the emerging idea that the atmosphere and ocean both play important roles in the complex set of processes that tend to be lumped together as “climate change.”

  14. Follow the Template

    Cytokines bind to the receptor gp130 and activate a variety of intracellular responses. Chow et al. (p. 2150) have determined a 2.4 angstrom-resolution structure of the ligand-binding and activation domains of gp130 bound to Karposi's sarcoma-associated herpesvirus interleukin-6 (IL-6). A tetrameric signaling complex is formed by interactions between viral IL-6 and the activation domain of human gp130. This assembly reveals why the activation domain is required for formation of a competent signaling complex. The tetramer provides a template to model the hexameric signaling complexes formed between gp130 and human cytokines and shows how the viral cytokine achieves molecular mimicry.

  15. Dual Function Enzyme

    DNA polymerase iota (pol ι) belongs to a growing superfamily of eukaryotic polymerases, many of which show low fidelity but whose precise cellular functions remain to be established. In experiments with purified proteins, Bebenek et al. (p. 2156) show that in addition to its polymerase activity, human pol ι has 5′-deoxyribose phosphate lyase activity, a functional feature normally associated with base excision repair (BER). Indeed, in reconstituted reactions with other BER enzymes, pol ι participated in repair of G-U and A-U base pairs. These data raise the possibility that pol ι may play a role in specialized forms of BER in vivo.

  16. O2- Ions and the Mars Labeled Release Response

    Reporting on experiments under simulated martian conditions, Yen et al. (Reports, 15 Sep. 2000; p. 1909) argued that formation of superoxide radicals constitutes “the most straightforward explanation” for the martian soil's unusual reactivity and for the apparent absence of organic molecules at the martian surface. Levin comments that their results “differ sharply” from control data from the Viking labeled release (LR) life detection experiments, which showed a temperature-dependent inactivation of martian samples on heating, and that additional LR data not mentioned by Yen et al. challenge the notion that superoxides constitute the reactive agent in the martian soil, leaving intact the possibility that “living microorganisms were detected in the LR experiment.” Yen et al. respond that the heated soils in the Viking experiments “released up to 0.2 weight percent water”—more than enough to scavenge reactive oxygen species—and propose alternative explanations for the additional data cited by Levin. The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/291/5511/2041a